This invention relates to rubber vulcanisates having improved physical properties.
The process of vulcanising diene rubbers by heating with sulphur and a vulcanisation accelerator has been known for many years. By this process vulcanisates having certain physical properties, for instance tensile strength, resilience and fatigue resistance at a high level can be obtained, but such vulcanisates tend not to have good ageing properties. Apart from the addition of antioxidants which will retard oxidative heat ageing, other methods which have been proposed for making vulcanisates having improved ageing properties include the use of lower proportions of sulphur and increased proportions of accelerator relative to those which would be employed for a conventional cure, and the partial or complete replacement of sulphur by other cross-linking agents. Examples of such cross-linking agents include amine disulphides, for example, N,N'-dithiodimorpholine, bis(sulphenamides) as described in GB Patent Specification No. 1,409,953 and U.S. patent specification No. 3,847,880, and compounds comprising two or more accelerator groupings linked through an organic bridging group as described in GB Patent Specification No. 1,388,279.
Vulcanisates made using such alternative systems tend, however, to lack certain of the merits of a sulphurcured vulcanisate. For example, lowering the ratio of sulphur to accelerator or replacing the sulphur partially or completely by an amine disulphide, gives vulcanisates having inferior dynamic properties. The use of the aforementioned bis(sulphenamides) and compounds containing two or more accelerator groupings means that molecular species having accelerator activity as well as those having cross-linking activity are released into the vulcanising system, so that the freedom for variations in compounding, which is possible when the cross-linking agent and the accelerator are added as separate entities, is lost.